Yeah, it's completely ethical to steal from big corporations. They steal from you every single day. Their entire existence is based on the amoral exploitation of other humans. Other people, small businesses, a very very very few medium-large corporations, it's not ethical to steal from, they're not a disgusting blight rotting away the foundations of society. Nor a friend, or stranger, or small business, etc. for the same reason.

I feel like this isn't a particularly hard concept.